1. Field of the Invention
This invention relates to the field of silver-iron batteries and, more particular, to a method for forming a silver-iron multi-cell bipolar battery and a silver-iron jelly roll battery cell.
2. Description of Related Art
The silver-iron battery cell is a stable, rechargeable electrochemical system that has a long cycle life and is capable of relatively high rates of charge and discharge. Brown, U.S. Pat. No. 4,078,125 discloses a high energy density iron-silver battery wherein a silver electrode and an iron electrode were coupled together as an electrochemical system. Buzzelli, U.S. Pat. No. 4,383,015, discloses an iron-silver battery having a shunt electrode in which a silver electrode and an iron electrode were also coupled together as an electrochemical system. In each of these patents, a sintered iron (Fe) electrode was utilized. A sintered silver electrode was produced using either Ag powder or a Ag/Ag.sub.2 O powder mix. A separator system capable of retarding silver ion migration from the positive silver electrode to the negative iron electrode was also provided. These patents dealt primarily with prismatic, multiple-plate cell designs.
Most of the elements used in a prismatic, multiple-plate cell design are relevant to a bipolar system. However, the methods of fabricating bipolar electrodes are somewhat different than the methods used for prismatic design. The fabrication of silver-iron bipolar systems use both sintered iron and sintered silver electrodes. The sequence and methods for ultimately bonding these electrodes to a single bipolar structure can be problematic.
FIG. 1 shows a bipolar battery 10 comprised of two or more bipolar cells 12, 14, stacked in series in case 15. The unique feature of the bipolar construction is that the anode 16 of one cell 14 is in direct electrical contact with the cathode 18 of the next cell 12 in the series through a metal sheet called the bipolar plate 20. Bipolar plate 20 serves as the intercell connector, the current collector, the barrier which prevents electrolyte communication from one cell 12 to the next 14, and the substrate to which the electrochemically active material (silver or iron) must be bonded during manufacturing.
The fabrication of bipolar battery 10 includes the provision of end plate 22 to which negative terminal 24 is electrically connected and end plate 26 to which positive terminal 28 is electrically connected. An anode 30 is provided along end plate 26 and a cathode 32 is provided along end plate 22. Separators 34 are provided between each of the electrodes in each of cells 12 and 14. Finally, electrolyte 36 is provided in each of cells 12 and 14.
In fabricating bipolar battery 10, bonding the active electrode materials to the bipolar plate 20 is a difficult task. One method commonly used is to press and sinter bond iron (Fe) powder onto one side of the plate at 700.degree.-800.degree. C., and then press and sinter bond Ag or Ag.sub.2 O powder onto the opposite side of the plate at 400.degree.-500.degree. C. During the first sintering process, the iron remains in the Fe state, which is the electrochemically charged state of iron. During bonding of the silver material, however, all silver species are reduced to the Ag state, which is the electrochemically discharged state of the silver. Subsequently, when the cell stack is assembled, it must go through an electrochemical silver formation charge to convert Ag to Ag.sub.2 O. Only when the silver anode is in the form of Ag.sub.2 O may the cell then be discharged.
The problem with this commonly-used approach is that H.sub.2 gas evolves from the already-charged iron electrode during the initial Ag formation charge. This evolution of gas causes the cells to dry out. Because the bipolar cells are designed to be packed very tightly together with only a minimum of space between electrodes, the amount of electrolyte provided in the cells is also kept to a minimum. The gassing at the iron electrode starves the cells of electrolyte, which greatly limits the effectiveness of the battery in subsequent cycles. This problem is not encountered in prismatic cell designs where plates are not stacked so tightly together and where cells are flooded with excess electrolyte by design.
The largest obstacle to the development of a working silver-iron bipolar battery has been the technique for applying the silver active material to the bipolar plate. Prior to the present invention, the commonly-used techniques have resulted in coupling of a fully charged iron electrode and a fully discharged silver electrode. Consequently, there is a need for a method to fabricate a bipolar plate which results in the coupling of a fully charged iron electrode and s fully charged silver electrode.
Commercial secondary alkaline batteries are presently being manufactured in two basic design configurations, the rectangular prismatic design discussed above and the cylindrical jelly-roll design. Nickel-cadmium, lead-acid, carbon-zinc, manganese-zinc, mercury-zinc, and the various lithium couples are the most prevalent electrochemistries used in the jelly-roll configuration. One method of constructing a jelly-roll battery cell is to roll flexible electrodes around a central removable dowel rod, thus producing a cylindrical core called a "jelly-roll". Different sized rolls produce the familiar A, AA, C, and D cells that support an enormous consumer market for power tools, flashlights, cameras, computers, radios, and the like. These cells weigh only ounces and supply relatively low power and capacity.
The prismatic design can be constructed in much larger sizes than the jelly-roll cell and therefore can support much greater power and energy requirements. Electrochemistries such as silver-zinc, nickel-iron, and silver-iron are examples of cells that are typically produced in the prismatic configuration. However, several of the electrochemical couples used in the jelly-roll configuration are also manufactured in the prismatic configuration, such as nickel-cadmium and lead-acid. Prismatic cells are made up of flat plates, either the pocket or sintered types, which are stacked face-to-face thus producing a rectangular-shaped cell. They are operated in a flooded electrolyte condition, and require special venting considerations. Applications for large prismatic designs range from 50 pound batteries for cars and lift vehicles to batteries weighing tons for use on submarines.
By far the most successful electrochemistries from a commercial standpoint are the lead-acid and nickel-cadmium couples. It is no coincidence that both of these electrochemistries have been successfully engineered in both the jelly-roll and prismatic designs, thus covering essentially all possible markets.
Silver-iron couples have only been constructed in the prismatic configuration. The rigid nature of the sintered silver and iron electrodes has restricted their use to a flat-plate design.
Judging from the commercial success of the nickel-cadmium and lead-acid batteries, it is desirable to manufacture cells in both the prismatic and jelly-roll configurations. In order to adapt the silver-iron system from a prismatic design to a cylindrical jelly-roll design, the sintering step in the manufacture of both the silver and iron electrodes has to be eliminated. By eliminating the sintering step, the electrodes would be be more malleable or pliable, and capable of being rolled into a jelly-roll configuration. However, this has to be accomplished while retaining structural integrity and maintaining electronic contact of active materials to current collector grids.
Wet-pasting techniques such as those disclosed by Folser in U.S. Pat. No. 4,132,547, have been used for producing iron electrodes. However, such wet-pasting techniques are for pasting iron oxide (Fe.sub.2 O.sub.3) which has to be reduced to Fe, and in the process sintered to form a rigid plaque. No wet-pasting methods of an iron electrode manufacture are known to produce a flexible plate. Accordingly, there is a need for a method to produce a pliable, malleable electrode of iron in the fully reduced state and silver in the Ag.sub.2 O or Ag states.